1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in exhaust gas discharged from internal combustion engines for automobiles (mainly diesel-driven), boilers and the like, and a method for purifying exhaust gas using the foregoing. The present invention is particularly focused on NOx purification in an oxygen-excessive region.
2. Description of the Related Art
In recent years, there has been a large demand for low-fuel-consumption automobiles in view of the problems of the exhaustion of petroleum resources and global warming, and development of lean-burn automobiles has drawn attention. In lean-burn automobiles, exhaust gas atmosphere in a lean-burn driving mode becomes oxygen-excessive atmosphere (lean) as compared to a state of a theoretical air-fuel ratio. However, if a usual three-way catalyst is used in such a lean region, there has been a problem of an insufficient NOx purifying operation affected by excessive oxygen. Accordingly, the development of a catalyst capable of purifying NOx even in oxygen-excessive conditions has been long awaited.
In this context, various catalysts for purifying NOx in the lean region have been proposed heretofore. For example, there is proposed a catalyst which stores NOx in a lean region and purifies NOx by discharging NOx in a stoichiometric or rich condition, as represented by a catalyst loading platinum (Pt) and lanthanum (La) on a porous carrier thereof (Japanese Patent Application Laid-Open No. 5-168860 (1993)).